This was one of the fussiest and most difficult procedures ever. I tried looking around online to see how other people do it. I found one guy who sort of used this approach and it seems to provide the most consistently shaped rows, so that's what I picked.
Buy yourself some green foam at the fabric store. The stuff is super expensive. Why?! We had a 50% coupon, so that helped.
But I bought 2" thick foam when we only really needed 1" of thickness. Turns out, a bandsaw can resaw green foam perfectly! :) So that's what I did.
I made a sliding till that fits into one of the medium-sized drawers. It's 10" long by 6" wide by 2" deep. I cut a piece of hardboard to fit perfectly inside that till. Then I used spray adhesive to attach the foam to the hardboard. The guy online didn't do this, but I figured it was an important step and I'll explain why later.
Now that you have green foam attached to a solid backer board, you can mark out where you're gonna cut your rows. Use a Sharpie, you're not gonna see any of the marks anyway as they'll get covered with velvet.
A serrated bread knife cuts green foam much better than a razor knife does. Use your straight edge and cut through the entire thickness of the foam, right down to the backer board.
I spaced my rows 1" apart, and they're 1" deep.
Now comes the fussiest, most frustrating thing you're ever going to do.
It starts out simply enough. Cut a piece of velvet so that it's wide enough to wrap 1" onto the backer board, 1-1/8" up the side, and 6" over the face of your foam, 1-1/8" down the other side, and 1" onto the backer board again. Easy peasy.
Keep the fabric long. You don't really know how much you're going to need for this.
I wish I had more photos of this process, but it really took all of our concentration and all 4 hands at all times, so there wasn't really any opportunity to grab the camera.
We initially tried using spray adhesive on the foam and then shoving the velvet into each groove, but the fabric sticks before you have it placed where you want it and you end up with lumps and crooked parts. Glue is not the answer here.
We found the stapler to be a much better option...and it's mostly undo-able, as long as you hide those staple holes when you reposition the fabric to try again.
So here's our approach:I honestly couldn't do this by myself. I had to have Carolyn hold one straight edge at a time while I wielded the stapler.
Carolyn: Take 1 straight edge, preferably 1" wide, and place it into a groove. This will help you fold the foam down and out of the way so you can get the fabric to the bottom of the groom easily.
Steve: Fold the sheet of velvet in half, lengthwise with the velvet side in. Put another straight edge into that fold, so that gravity is holding the fabric taught.
Carolyn: lean the foam straight edge over, compressing the previous row of foam to get it out of the way.
Steve: Shove the fabric-wrapped straight edge into the groove.
Carolyn: Slide the foam straight edge out of the way. We don't need it anymore.
Carolyn: Take over holding the fabric-wrapped straight edge. Hold it firmly in place.
Steve: Carefully smooth the fabric out onto the foam so that it's not applying any downward tension to the foam. You just want to wrap the foam without compressing it. Compressing the foam will introduce irregularities between rows, or within a single row, as you might compress one part of the foam more than another. The goal here is to just wrap the foam as it exists, but not so loosely that you have excess fabric.
Carolyn: Continue applying firm downward pressure to the fabric-wrapped straight edge, but now lean it over so that the next row of foam is compressed and out of the way so Steve can reach in with the stapler.
Steve: Gently scoot the fabric-wrapped straight edge 1/8" out of the way. While holding the previous row of foam out of the way with your left hand, staple at the very bottom of the groove with your right hand. 4 staples across 6" was enough to hold the fabric evenly and neatly.
This photo is an exaggeration of what I'm trying to describe, where the grey layer is the velvet, the green is the foam, and the brown is the backer board.
Carolyn came up with a nice, neat way to fold the fabric when wrapping it around the hardboard and I just stapled it in place on the opposite side to hold it there. Since the staples punched through the hardboard, I just used a flat screwdriver to clinch them over.
Apologies for the photo quality. It's hard to get a clear picture with the fabric so dark.
PRO TIP: Make your hardboard about 1/8" narrower and shorter than the interior length and width of the space it's going to occupy. The folds of fabric wrapping around the hardboard will fill up this gap. If you cut the hardboard to fit perfectly, it won't fit once it has the fabric on it. This is another reason for adhering the foam to the backer board. Since the foam is glued down, it can't buckle when it gets compressed due to the extra fabric causing a tight fit. The result is a nice, flat, fabric-lined block of foam.